A pentaerythritol-tetrakis(3-alkylthio-propionate) is known to be a modifier or a thermal stabilizer for improving resistance of plastics, e.g., polyolefin resin, ABS resin, etc., to deterioration due to heat or copper in U.S. Pat. No. 3,629,194. One of known methods for preparing this compound comprises reacting an .alpha.-olefin with .beta.-mercaptopropionic acid or its ester and then reacting the resulting 3-alkylthiopropionic acid with pentaerythritol as taught in Japanese Patent Laid-Open Application No. 59225/79 and Japanese Patent Publication Nos. 39249/80 and 41654/80. However, this process produces not only n-alkyl compounds but also iso-alkyl compounds. Mixtures containing the isoalkyl compounds have greatly lowered melting points and require an increased cost for separation of the n-alkyl compounds.
U.S. Pat. No. 3,758,549 also discloses a process for preparing 3-alkylthiopropionic acid esters starting with alkylmercaptans. In this patent, however, there is only one specific example wherein ethylene glycol was used for esterification. A reference to pentaerythritol-tetrakis(3-n-dodecylthiopropionate), one of pentaerythritol-tetrakis(3-alkylthio-propionate), is found therein, but the information obtained from this reference is merely that this specific ester has a melting point of from 47.degree. to 49.degree. C.